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Maize (Zea mays L.) yields obtained by small-holder farmers in semi-arid zones in sub-Sahara Africa (SSA) are often less than half of potential yields. Water deficit during critical crop growth stages together with low nutrient input interacts to reduce yields. Collection of surface run-off, which could be used as supplemental irrigation may prove beneficial in improving current small-holder farming system in SSA. This paper presents the results of an on-farm study of the effects of supplemental irrigation (SI) on maize yield in semi-arid Kenya. Surface run-off from a catchment of 2.7 ha was harvested in a hand-dug earth dam of 300 m(2). The water was supplied by gravity to mitigate dry spells in fertilized (S130, S180 kg N ha(-1)) and non-fertilized (SI0 kg N ha(-1)) maize. Treatments of SI were compared to non-irrigated treatments (NI80, NI30, NI0 kg N ha(-1)). Rainfall varied, during the five seasons of study, from 196 to 564 mm. The volume of water harvested in the dam ranged between 1% and 4% of seasonal rainfall. The outtake for supplemental irrigation varied between 20 and 240 mm per season. Seepage losses accounted for 11 to 74% of harvested dam water. Lowest maize yields were in NI0, representing farmers’ current practise. SI with fertilizer increased yields compared to non-irrigated and fertilised treatments (NI30, NI80) for low rainfall seasons (< 300 mm). High rainfall seasons (> 300 mm) resulted in no yield increase for Sl compared to NI. Mean seasonal grain yield for SI and fertilizer (30 or 80 kg N ha(-1)) of 1796 kg ha(-1) was significantly higher (P < 0.001) than NI0 kg N ha(-1) of 1319 kg ha(-1), and higher than SI0 kg N ha(-1) and NI30 kg N ha(-1) (P < 0.01). Lowest average rain and irrigation water use efficiency (RUE, kg grain mm(-1) ha(-1)) was for NI0 with RUE = 2.1, and highest for SI30 with RUE = 4.1. Water harvesting of surface run-off added as SI resulted in improved maize yields as a result of dry spell mitigation, but only in combination with N fertilizer. To upgrade on-farm water management in semiarid SSA, the results suggest that supplemental irrigation combined with fertilizer may reduce the currently existing yield gap in small-holder farming systems. (c) 2004 Elsevier B.V. All rights reserved.